Transparent NullMoney operations

Operations relative to other MoneyInterfaces objects are now
transparent (they do not return $this, but the comprared Money object).
This avoids that issuing such operations between Money and NullMoney
would return NullMoney instead of Money objects.

Setters were also deleted from MoneyInterface, since objects
implementing this interface will be value objects.

See issue #292 
This should return an integer. Which one? Is a `NullMoney` **always** lesser, equal or greater than any given `Money`? Maybe we can treat `NullMoney` in this case as a `Money` with `amount = 0`, so it should `return $other->compareTo(new Money(0, $other->getCurrency());`